Craniofacial Muscles

248 Z.-J. Liu
various dimensions (width, length, and thickness) in a region of the tongue may not
be parallel or compensatory to each other, thus incompatible with the hydrostat
theory. Our 3D digital ultrasound recording demonstrated that rhythmic and stereotypical
regional volumetric changes do occur during mastication and drinking, and
these volumetric changes are signi fi cantly larger in chewing (~45.6%) than in ingestion
(~31.4%) and drinking (~30.4%). The regional volumetric expansion mainly
results from widening and shortening, and posterior thinning in the tongue body.
The data also suggest that the theory of region-independent motor control of the
tongue (Slaughter et al . 2005 ; Hiiemae and Palmer 2003 ) , i.e., one dimension of the
tongue compensates for the other dimension or the loss of one dimension is parallel
to the gain of other dimension, may not really occur. The time-series analyses
between the dimensional and volumetric changes further revealed that the volume
expansion is primarily due to the increase of widths while thickness and length actually
decrease. If the overall changes in amplitudes of various dimensions are
counted, decreases in thickness and length are the two biggest contributors to volumetric
expansion. Therefore, regional volumetric changes are coupled with changing
widths in the same direction and with changing thickness and length in the
opposite directions (Liu et al . 2008c ) .
14.2.4 Tongue Kinematics in Relation to Jaw Movement
and Muscle Activity
Tongue kinematics are restricted neither to the simple protrusion–retrusion and/or
descending–ascending axes, nor to vertical rotation like that seen for the jaw, but
involves complex shape and regional volumetric changes during function. These
changes are produced by sequential muscular activity and accompanied by jaw
movement (Hiiemae et al . 1995 ; Napadow et al . 1999a, b, 2002 ; Liu et al . 2008c ) .
A number of studies have indicated a strong linkage between tongue and jaw movements
during feeding (Liu et al . 1993 ; Thexton et al . 1982 ; Palmer et al . 1997 ) , but
the linkage between internal deformation of the tongue and jaw movement is largely
unknown. High-speed video with fl uorescent markers glued to the lips and the
tongue tip revealed that during mastication, the tongue tip retracts when jaw opening
begins, with a time delay of 16–24 ms (Fig. 14.6 ). This very rapid movement
most likely relates to the control of the bolus, and this observation contradicts the
widely accepted notion that tongue protrusion coincides with jaw opening during
rhythmic chewing (Thexton et al. 1982 ; Liu et al. 1993 ; Palmer et al. 1997 ) .
Conversely, during jaw closing it is the corner of the mouth which retracts suggesting
that the cheek is responsible for guiding the bolus at this stage. The realtime
and synchronized study on tongue internal deformation, jaw movement, and
EMG activities revealed that expansion of tongue widths mainly occurs in the
occlusal phase of jaw movement and is less coupled with the activity of tongue
muscles, but the expansions of length and thickness are seen in the opening and
closing phases of jaw movement and are better coupled with activities of tongue